1. Field of the Invention
The present invention relates to a piezoelectric polymeric material consisting essentially of a copolymer of vinylidene fluoride and ethylene trifluoride and a process for producing the same in such a manner as to have a thickness extensional electromechanical coupling factor of at least 0.05, and further an ultrasonic transducer utilizing the same.
2. General Description of the Prior Art
It is well known that a polymer film may be made to be piezoelectric by having the film undergo an electrically polarizing (poling) treatment. Generally piezoelectric polymeric materials are preferred over inorganic single crystals or ceramic piezoelectric materials since piezoelectric polymeric materials not only have flexibility, and high water resistance and impact resistance, but their area and shape are not substantially restricted. Further, the acoustic impedance of a piezoelectric polymeric material is very close to those of water, plastics and living organisms. A piezoelectric polymeric material functioning as a transducer exhibits a low Q value upon transmission and reception of a sound wave to and from water, plastics, living organisms, etc. Therefore, as a broad band transducer piezoelectric polymeric materials are far superior to inorganic piezoelectric material for transducers for diagnosing the acoustic properties, especially the ultrasonic characteristics of substances such as plastics and living organisms.
One of the various factors indicating the performance of an ultrasonic transducer is the ratio of mechanical power output to electrical power input (or the ratio of electrical power output to mechanical power input) known as the electromechanical coupling factor. Of particular concern is the electromechanical coupling factor K.sub.t concerning thickness vibration.
The thickness extensional electromechanical coupling factor K.sub.t referred to herein is defined as follows: ##EQU1## wherein e.sub.33 : Piezoelectric stress constant
C.sub.33.sup.D : Elastic constant under fixed electric displacement PA1 .epsilon..sub.33.sup.S : Dielectric constant under fixed strain PA1 Suffix letter: Z(3) axis is selected perpendicular to the film surface and letters are added thereto. PA1 M1: The number average molecular weight. PA1 M2: The weight average molecular weight. PA1 M3: The Z average molecular weight. PA1 All units of M1, M2, and M3 should be multiplied by 10.sup.5.
The value of the K.sub.t is measured by applying a high frequency (1-50 MHz) an AC field at room temperature to an about 1 cm.sup.2 circular or square film and analyzing the frequency characteristics of an electric admittance Y in the vicinity of a resonance region of the film. As to the details, please refer to Journal of Applied Physics, Vol. 47, No. 3, March 1976, pages 949-955.
The terms transverse piezoelectric properties and thickness piezoelectric properties referred to herein are defined as follows. Namely, in a piezoelectric polymer film, the poling direction is perpendicular to the film surface and this direction is made the Z(3 ) axis. The axes of symmetry within the film surface are selected as the X(1) axis and the Y(2) axis. When the film is monoaxially drawn, this axis is made the X(1) axis. At this time, the components of the piezoelectric moduli or d-constants are as follows. ##EQU2## The piezoelectric constants shown by d.sub.31 and d.sub.32 are related to the transverse piezoelectric properties and the piezoelectric constant shown by d.sub.33 is related to thickness piezoelectric properties. For information, it is most effective to use the piezoelectric properties in the vicinity of a frequency where this thickness expansion and contraction vibration is substantially in a resonance state under circumstances decided by the electric and acoustic properties of the film per se and the boundary conditions of the film. However, as occasion demands, it is possible to use the piezoelectric properties at a non-resonance frequency.
The value of the piezoelectric constant d.sub.31 with respect to the transverse piezoelectric properties is sought by mechanically exciting a film at room temperature with 110 Hz by Vibron-II manufactured by Toyo Sokki Co., Ltd. of Japan and measuring the amount of AC electric charge generated on the film surface and the stress at the section of the film. Of previously known piezoelectric polymeric materials, that material have the highest performance is a poled polyvinylidene fluoride (hereinafter referred to as PVDF) film. However, according to the study of the present inventors, the K.sub.t of PVDF is about 0.2 at present if it is poled under good conditions, which is considerably smaller than the K.sub.t of piezoelectric ceramic material. It is difficult to prepare PVDF having a larger K.sub.t. Further, PVDF exhibits large dielectric loss and mechanical loss in the important working frequency range in practical use (not less than 1 MHz) of an ultrasonic transducer. Thus, PVDF not only generates heat when it is electrically driven, but also exhibits a lower efficiency. For these reasons, a piezoelectric ultrasonic transducer using PVDF is not ideal and the identification of a more efficient transducer material has been earnestly desired.
The piezoelectric properties of a polymeric material consisting of ethylene trifluoride and vinylidene fluoride P(VDF-TrFE) is described in Laid-open Japanese Patent Application No. 26995/1978. Studies on the polymer were presented by Higashihata, Yagi, Sako on Page 325; Uchidei, Iwamoto, Iwama, Tamura on Page 326; and Yamada, Ueda, Kitayama on Page 326 of Manuscripts of speeches at the meeting of the Applied Physics Society (April 1979 at Tokyo). These disclosures related to the piezoelectric properties (d.sub.31) concerning the expansion and contraction in a direction parallel to the film surface.